The OP is the best judge of whether the comment is obsolete, and has no particular incentive to be unscrupulous in leaving an old comment - and even if they did, chances are this feature will lead to more obsolete comments being cleaned up than status quo in my estimation

Right now there's no real way for two users to jointly clean up an obsolete comment thread. If I delete mine, I need to let the conversation partner know they can cleanup their half... but then my "notification" comment remains, and there's in general no way out of this loop

If OP doesn't take action in, say, 2 days, can notify mods anyway

It proselytizes itself: if 100 people on meta see this thread, when they flag other comments as obsolete, they are educating the rest of the user base that it's okay to flag comments as obsolete. Plus there is already some badge-incentive regarding flags to help encourage people

It solves the problem we have that it's impossible for an OP to notify a downvoter (who comments) or commenter that the improvement has been made with no weight added to our UI

I don't know that I agree with your first bullet. I can envision plenty of cases where the OP may think that a comment/question has been addressed but the other person does not. I talked about this here, but you complained that I wrote too much. :-\
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Aaron BertrandJun 10 '13 at 1:18

Your bullet points don't state explicitly the obvious win that it also helps reduce the moderator workload. And so should be welcomed per the thread here. I imagine most regulars would happily clean up their own obsolete comments if notified of them which could cause a significant reduction in amount going through for moderator handling.
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Martin SmithFeb 18 '14 at 15:44